Safety and Immunogenicity of an Inactivated Severe Acute Respiratory Syndrome Coronavirus 2 Vaccine in a Subgroup of Healthy Adults in Chile

Abstract

Background: The development of effective vaccines against coronavirus disease 2019 is a global priority. CoronaVac is an inactivated severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccine with promising safety and immunogenicity profiles. This article reports safety and immunogenicity results obtained for healthy Chilean adults aged ≥18 years in a phase 3 clinical trial.

Methods: Volunteers randomly received 2 doses of CoronaVac or placebo, separated by 2 weeks. A total of 434 volunteers were enrolled, 397 aged 18-59 years and 37 aged ≥60 years. Solicited and unsolicited adverse reactions were registered from all volunteers. Blood samples were obtained from a subset of volunteers and analyzed for humoral and cellular measures of immunogenicity.

Results: The primary adverse reaction in the 434 volunteers was pain at the injection site, with a higher incidence in the vaccine than in the placebo arm. Adverse reactions observed were mostly mild and local. No severe adverse events were reported. The humoral evaluation was performed on 81 volunteers. Seroconversion rates for specific anti-S1-receptor binding domain (RBD) immunoglobulin G (IgG) were 82.22% and 84.44% in the 18-59 year age group and 62.69% and 70.37% in the ≥60 year age group, 2 and 4 weeks after the second dose, respectively. A significant increase in circulating neutralizing antibodies was detected 2 and 4 weeks after the second dose. The cellular evaluation was performed on 47 volunteers. We detected a significant induction of T-cell responses characterized by the secretion of interferon-γ (IFN-γ) upon stimulation with Mega Pools of peptides from SARS-CoV-2.

Conclusions: Immunization with CoronaVac in a 0-14 schedule in Chilean adults aged ≥18 years is safe, induces anti-S1-RBD IgG with neutralizing capacity, activates T cells, and promotes the secretion of IFN-γ upon stimulation with SARS-CoV-2 antigens.

Keywords: COVID-19; CoronaVac; SARS-CoV-2; phase 3 clinical trial; vaccines.

Figure 1.

Study profile. Recruitment of volunteers for the phase 3 clinical trial as of February 10, 2021.

Figure 2.

Immunization with CoronaVac induces specific IgG against SARS-CoV-2 antigens in participants aged 18–59 years and ≥ 60 years after 2 immunizations in a 0–14 schedule. Titers of IgG antibodies after 2 doses of CoronaVac were evaluated for immunized participants (excluding seropositive participants at recruitment and placebo participants) before the first and second dose, and 2 (second dose + 2 weeks) and 4 weeks (second dose + 4 weeks) after the second dose for adults aged (A, C) 18–59 years and (B, D) ≥60 years. Specific IgG against the S1-RBD (upper panel) and the N protein (lower panel) of SARS-CoV-2 were measured. Data are expressed as the log₁₀ of international WHO arbitrary units versus time after each dose. Error bars indicate the 95% CI of the geometric mean units (GMUs). The spots represent the individual values of antibody units for each volunteer, with the numbers above each time showing the GMU estimates. The graph illustrates the results obtained for 45 participants in the 18–59 years group and 27 participants in the ≥60 years group. One-way ANOVAs with repeated measures and post hoc Tukey tests were performed to evaluate statistical differences among the groups; *P < .05, **P < .005, ***P < .0005, ****P < .0001. Abbreviations: ANOVA, analysis of variance; CI, confidence interval; IgG, immunoglobulin G; N, nucleocapsid; RBD, receptor binding domain; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2; WHO, World Health Organization.

Figure 3.

Immunization with CoronaVac induces neutralizing antibodies against SARS-CoV-2 in participants aged 18–59 years and ≥60 years after 2 immunizations in a 0–14 schedule. (A-B) Neutralizing antibody titers were evaluated with a surrogate virus neutralization assay, which quantifies the interaction between the S1-RBD and hACE2 precoated on enzyme-linked immunosorbent assay plates. Results were obtained from (A) 45 participants aged 18–59 years and (B) 27 ≥ 60 years before the first and second dose, and 2 (second dose + 2 weeks) and 4 weeks (second dose + 4 weeks) after the second dose. (C-D) Titers of neutralizing antibodies were evaluated with a pseudotyped viral system. Data are represented as the reciprocal dilution of sera that prevented infection by 80% (ID80) after the first dose. Numbers above the bars show the geometric mean titer (GMT), and the error bars indicate the 95% CI. Results were obtained from 45 participants (C) aged 18–59 years and (D) 24 ≥ 60 years before the first and second dose, and 2 (second dose + 2 weeks) and 4 weeks (second dose + 4 weeks) after the second dose. (E-F) Titers of neutralizing antibodies evaluated with a conventional neutralization assay using an ancestral D614G variant strain of SARS-CoV-2. Data are represented as the reciprocal dilution of sera that prevented infection after the first dose. Numbers above the bars show the GMT, and the error bars indicate the 95% CI. Results were obtained from 45 participants aged (E) 18–59 year and (F) 27 ≥ 60 years before the first and second dose, and 2 (second dose + 2 weeks) and 4 weeks (second dose + 4 weeks) after the second dose. Data are represented as the reciprocal antibody titer versus time after each dose. Numbers above the bars show the GMT, and the error bars indicate the 95% CI. Data were analyzed by a Wilcoxon test to evaluate statistical differences among the groups; *P < .05, **P < .005, ***P < .0005, ****P < .0001. Abbreviations: CI, confidence interval; RBD, receptor binding domain; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2.

Figure 4.

Evaluation of cellular immune response through ELISPOT upon stimulation with Mega Pools of peptides derived from SARS-CoV-2 proteins in volunteers immunized with CoronaVac. Numbers of IFN-γ-secreting cells, determined through ELISPOT as spot forming cells (SFCs) were determined. (A) Representative pictures for each stimulus are shown. PBMCs were stimulated with (B) MP-S, (C) MP-R, (D) MP-S + R, (E) MP-CD8A, (F) MP-CD8B, and (G) MP-CD8A + B for 48 h for samples obtained before the first dose, and 2 (second dose + 2 weeks) and 4 weeks (second dose + 4 weeks) after the second dose. A total of 47 volunteers were evaluated for MP-S and MP-R and 27 volunteers for MP-CD8A and MP-CD8B. Data shown represent median ± 95% CI. Statistical differences were evaluated by a Friedman test for repeated measures, followed by a post hoc Dunn test corrected for multiple comparisons against day preimmune samples; n.s. = no statistical differences, *P < .05, **P < .005. Abbreviations: CI, confidence interval; ELISPOT, enzyme-linked immunospot; IFN, interferon; MP, Mega Pools; PBMC, peripheral blood mononuclear cell; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2.

Figure 5.

Changes in activation-induced markers (AIMs) expression in T cells through flow cytometry upon stimulation with Mega Pools of peptides derived from SARS-CoV-2 in volunteers immunized with CoronaVac. (A) The gating strategy used to evaluate changes in the expression of AIMs upon stimulation of PBMCs is shown. PBMCs were stimulated with (B) MP-S, (C) MP-R, (D) MP-S + R, (E) MP-CD8A, (F) MP-CD8B, and (G) MP-CD8A + B for 24 h for samples obtained before the first dose, and 2 (second dose + 2 weeks) and 4 weeks (second dose + 4 weeks) after the second dose. Changes in the expression of AIMs for CD4⁺ T cells (OX40⁺ CD137⁺) were measured upon stimulation with (B) MP-S, (C) MP-R, and (D) MP-S + R. Changes in the expression of AIMs for CD8⁺ T cells (CD69⁺ CD137⁺) were measured upon stimulation with (E) MP-CD8A, (F) MP-CD8B, and (G) MP-CD8A + B. A total of 43 volunteers were evaluated for MP-S and MP-R and 21 volunteers for MP-CD8A and MP-CD8B. Data shown represent mean ± standard deviation. Statistical differences were evaluated by a Friedman test for repeated measures, followed by a post hoc Dunn test corrected for multiple comparisons against preimmune samples. n.s. = no statistical differences, *P < .05, **P < .005, ****P < .0001. Abbreviations: MP, Mega Pools; PBMC, peripheral blood mononuclear cell; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2.